Flotation



P. M. FRANTZ ET AL FLOTATION Feb. 16, 1943.

Filedneo. 17, 1941 iwf,

Patented Feb. 16, 1943 FLOTATION Philip M. Frantz and William M. Bertholf, Pueblo, Colo., assis-nots to The Colorado Fuel and Iron Corporation, Denver, Colo., a corporation of Colorado Application December 17, 1941, Serial No. 423,342

Claims.

This invention is concerned with froth .flotation and provides an improved skimmer for removing froth accumulating in a flotation machine. The invention is particularly applicable to the skimming of a fast-forming froth of coal and the like, but may be employed advantageously in the removal of all types of mineral-bearing froths. l

In froth flotation, an aqueous pulp of a comminuted mineral aggregate is subjected to aeration or the like in the presence of one or more flotation reagents with the result that a mineralbearing froth is formed on top of the pulp. Some flotation froths are such that they overflow from a discharge lip at or near the top of the pool of pulp without aid. However, in many cases it is necessary to skim the froth. When flotation froths are very viscous and do not flow readily, it is desirable to skim them. Fragile froths, which tend to break readily, advantageously are skimmed from the top of the pulp before the mineral floated has an opportunity to sink back into the pulp. Thirdly, when the mineral floated, is a large proportion of the total ore, the froth may form so fast that skimming is necessary to obtain maximum capacity for a given flotation machine.

It has been proposed heretofore to push froth over the discharge lip of a flotation machine with positively driven wheels mounted on a substantially horizontal axis and equipped with one or more flat paddles that skim the froth from the top of the pulp. However, such paddle Wheels are not suitable for skimming all types of froth. If the flat paddles are rotated at the high speed required When a fast-forming froth is to be skimmed, violent splashing and tossing may occur. In such case, some of the froth is broken so that its mineral content has to be re-floated. Even at slow speeds, the action of flat paddles of the type employed heretofore is uneven. Such i paddles slap the froth and tend to break it with consequent reduction in the capacity of the flotation machine.

The deleterious effect of the flat paddles is particularly serious in coal flotation Where the amount of froth fformed is very large in propordeveloped a novel skimmer adapted to operate in conjunction with a froth over-flow for a froth flotation machine'and which is muchv more efllcient than the flat paddle type. It comprises a helical paddle (preferably of steep pitch) rotatably mounted on an approximately horizontal axis above the pulp pool of a froth flotation machine with its lower edge in the neighborhood of the level of the pool, i. e., so that the periphery of the rotating blade scrapes substantially horizontally at or near the pulp level. In the preferred form, the axis of rotation is horizontal and the paddle comprises a pair of blades of uniform width measured from their periphery to the axis. The paddle is positively driven and should be mounted reasonably close to the discharge or over-flow of the flotation machine and preferably substantially parallel thereto, so that as the helix is rotated it cuts gradually into the froth bed overlying the pool beginning at one end and continuously pushes the froth over the discharge.

In short, our invention contemplates the combination in a froth flotation machine having a flotation chamber adapted to contain a pool of liquid underlying a bed of mineral-bearing froth which comprises a skimming member in the form of a helical blade .rotatably mounted on an approximately horizontal axis above the pool, a froth overflow on the chamber adjacent the skimming member but below its axis, and means for positively rotating the helical blade about its axis.

The helical skimmer of our invention, in its preferred form, resembles a double-thread screw with a very steep pitch or a lawn mower equipped with a single blade that extends from the shaft to the periphery.

These and other features of our invention will be more thoroughly understood in the light of the following detailed description taken in conjunction with the accompanying drawing in which:

Fig. l is afragmentary elevation, partly in section, of the spitzkasten or frothing chamber of a flotation machine equipped with an improved skimmer of our invention;

Fig. 2 is a longitudinal elevation of the skimmer of Fig. 1; and

Fig. 3 is a plan view of the skimmer.

Referring to the drawing, it Will be observed that the spitzkasten I0 of a froth flotation machine is filled with an aqueous ore pulp Il to a level I2 slightly below the top of a discharge lip I3 at the top of the spitzkasten. When the pulp in the machine is aerated, a body offfroth I4 is formed on the top of thepulp and this froth lis pushed over the discharge lip by a skimmer mechanism I5. The skimmer mechanism comprises a rotatably mounted horizontal shaft I6 that is disposed substantially parallel to the discharge lip. A helical paddle Il of screening, sheet metal or the like, is fastened concentricallyv around the shaft, the pitch of the helix being such that it makes about one-third of a complete turn about the shaft in the length of the helix, which extends lengthwise of the flotation machine. The shaft is positively rotated in acounterclockwise direction (as viewed in Fig. 1) by means of a of required re-iiotation.

pulley il provided with a belt It or other conventional driving means.

Our preferred helical skimmer has two blades HA, I'IB mounted opposite each other on the shaft, so that it resembles a double-thread screw on which the edge of each blade corresponds'to a thread of the screw. However, the skimmer may be provided with more than two blades, somewhat in the manner of a lawn mower, but ordinarily this is unnecessary and we prefer to employ a summer with a single blade.

As indicated above, the pitch of the helix Apreferabiy is quite steep. Tests indicate that optimum results are obtained when the summer plate makes about one mplete turn around its shaft `in a length of 36", or approximately onetblrd of a complete turn in its own length, say, about 12". The rule as to the number of turns in terms of length appears to be applicable, at least within broad limits, irrespective of the width of the plate. In other word', a plate having a radius of 6" should have about the same turns per unit of length as a plate having a radius of, say, 12".

Comparative tests with summers of the conventional iiat paddle -type show the great superiority of theskimmer of our invention. In the tests, the flat paddle skimmer was exactly the same as the helical skimmer in dimension, location and material except that in the helical skimmer the plate was twisted about its axis with a pitch of about one-third of a turn in l2". The two summers were subjected to tests imder identical conditions in the skimming of a coal-bearing froth produced with a mineral separation" type of flotation machine commonly used for coal flotation and provided with a double discharge.

It was found that the fiat paddle type of skimmer caused excessive splashing and tossing of the froth when it was operated with a peripheral speed (measured at the tips of the paddle) exceeding about sixty feet per minute. Moreover,

the summing action of the fiat paddle was lsomewhat intermittent so that the froth came to rest between revolutions of the paddle with a resultant slapping action which tended to break the froth. The broken froth represented mineral which had to be re-iloated with a consequent decrease in the capacity of the flotation machine In contrast, the helical skimmer of our invention could be run at peripheral speeds more than twice the maximum of the fiat paddle without causing undue splashing. Moreover, the skimming action of the helixwas much smoother since it tended to cut into the froth bed gradually from side to side. 'I'here was, therefore, a minimum of froth destruction and a minimum It was found that the helical skimmer could be operated over a considerable velocity range with good results. Thus, a helical skimmer having a radius of about 6" could be operated over a velocity range oi' from about 16 R. P. Mfto about 85 R. P. M. y

In the tests, it was established that the helical paddle rotating at the same speed as the conventional flat paddle would remove approximately twice as much froth in a given time, or, otherwise expressed, would remove a given quantity of froth in about one-half the time required by the fiat paddle. However, because the helical blade can be rotated at speeds approximately twice the maximum of the flat paddle without bringing about serious splashing and tossing, a helical blade operating at its maximum speed will remove about three times as much' froth 'in a given time as a flat paddle vof `egual sine operating at its maximum speed.

The tests indicate that in the notation of a material like coal where the concentrates amount to a large proportion. say I5 to 85% of the feed. and the ilotaticntime is short, the use of helical paddles will nearly double the capacity of a given flotation machine, for example a M S." machine with double froth discharge.`

The helical blades of our invention may be made of a variety of materials. Smooth sheet metal and screen cloth are both satisfactory. although the sheetmetal `is preferred. In the flotation of coal. or in other froth notation grocesses in which the summer may be subjected to corrosive influences, we prefer to employ a helical blade of stainless steel or other corrosionresistant metal.

We claim:

l. In a froth notation machine having a flotation chamber adapted to contain a pool of liquid underlying a bed of mineral-bearing froth, the combination which comprises -a member in the form of a helical blade rotatably mounted on an vapproximately horizontal axis above the pool, a froth over-flow on the chamber adjacent the skimming member and below its axis, and means for positively rotating the helical blade about its axis.

2. In a froth flotation machine having a flotation chamber adapted to contain a pool of liquid underlying a bed ofmineral-bearing froth, the combination which a skimming man berinthe formoi'asteeplypitchedhelicalisladel rotatably mounted on an approximately tal axis above the pool. a froth over-flow on the chamber adjacent the summing member but below its axis, and mean for positively ,rctatim the l helical blade about its axis.

3. Inafrothflotationmachinehavingaflbta tion chamber adapted to contair. a pool of liquid underlying a bed of annual-bearing froth, the combination which c a froth over-flow on the chamber, a summing member in the formy of at least one helical blade rotatably mountcd on. an approximately horizontal axis abone thepool and the overflow and approximately parallel to the froth over-flow, and means for positively rotating the helical blade about its axis.

i. In a froth notation machine vhaving a flotation chamber adapted to contain a poc'. oi liquid underlying a bed of mineral-bearing froth, the combination which comprises a mm1- ber in the form of a'pair'of helical'blades rotatabLv mounted opposite each other om an approximately horizontal axis above the pool. a froth over-flow on the chamber adjacent thel member but below its axis. and means for waitiveiy rotating the helicdl biedelbont its nail.'

5. In a froth iintationmachine having anotation chamber adapted to contati-a pool oflimtid underlying a bed of minerai-'bearing froth, the

combination which comprises ya. summing memberinthei'ormofatleastonehelicalbladerotatably mounted on an approximately horizontal axis above the pool and so pitched that the l'iiadeV makes approximately one-third of a turn about its axis in its length, a froth over-flow on the chamber adjacent the member but helow its axis, andmeansforpositively rotating the helical blade about its axis.

PHIIII? M. WIILIAM M. `BERI'II Ll". 

